There are many extensions that I am using very much. So here are some comments about some of the extensions that are selected for the final contest.
Adblock by Ben Karel
Block ads including Flash ads from their source. Right click on an ad and select Adblock to block ads. Hit the status-element and see what has or hasn't been blocked.
Firefox Showcase by Josep del Rio
View open Tabs and Windows with Showcase. You can use it in two ways: global mode (F12) or local mode (Shift + F12). In global mode, a new window will be opened with thumbnails of the pages you've opened in all windows. In local mode, only content in tabs of your current window will be shown. You can also right click in those thumbnails to perform the most usual operations on them. Mouse middle button can be used to zoom a thumbnail, although other actions can be assigned to it.This is a pretty useful extension which shows thumbnails of all the pages that you have loaded currently. I like this extension.
I think one of the greatest extension is missing in the finalist list. It is the Tab Mix Plus.
Tab Mix Plus 0.3.0.4, by CPU
Tab Mix Plus enhances Firefox's tab browsing capabilities. It includes such features as duplicating tabs, controlling tab focus, tab clicking options, undo closed tabs and windows, plus much more. It also includes a full-featured session manager with crash recovery that can save and restore combinations of opened tabs and windows.
This is one of the greatest extensions I have ever used as it provide so many features to manage tabs.
If you think Internet Explorer is great, read this